1. Field of the Invention
The present invention relates generally to micro-fluid ejection devices and, more particularly, to a heater stack in a micro-fluid ejection device and a method for forming a floating electrical heater element in the heater stack.
2. Description of the Related Art
Micro-fluid ejection devices have had many uses for a number of years. A common use is in a thermal inkjet printhead in the form of a heater chip. In addition to the heater chip, the inkjet printhead basically includes a source of supply of ink, a nozzle plate attached to or integrated with the heater chip, and an input/output connector, such as a tape automated bond circuit, for electrically connecting the heater chip to a printer during use. The heater chip is made up of a plurality of resistive heater elements, each being part of a heater stack. The term “heater stack” generally refers to the structure associated with the thickness of the heater chip that includes first, or heater forming, strata made up of resistive and conductive materials in the form of layers or films on a substrate of silicon or the like and second, or protective, strata made up of passivation and cavitation materials in the form of layers or films on the first strata, all fabricated by well-known processes of deposition, patterning and etching upon the substrate of silicon. The heater stack also has one or more fluid vias or slots that are cut or etched through the thickness of the silicon substrate and the first and second strata, using these well-known processes, serve to fluidly connect the supply of ink to the heater stacks. A heater stack having this general construction is disclosed as prior art in U.S. Pat. No. 7,195,343, which patent is assigned to the assignee of the present invention. The disclosure of this patent is hereby incorporated by reference herein.
Despite their seeming simplicity, construction of heater stacks requires consideration of many interrelated factors for proper functioning. The current trend for inkjet printing technology (and micro-fluid ejection devices generally) is toward ultra-low energy ejector designs that will provide lower jetting energy, greater ejection frequency, and in the case of printing, higher print speeds. However, a minimum quantity of thermal energy must be present on an external surface of the heater stack, above an electrical resistive heater element therein, in order to vaporize the ink inside an ink chamber between the heater stack external surface and a nozzle in the nozzle plate so that the ink will vaporize and escape or jet through the nozzle in a well-known manner. With current designs, the overall heating energy or “jetting energy” produced by the heater element must pass through the plurality of layers of the first and second strata that form the heater stack before the requisite energy for fluid ejection reaches the external surface of the heater stack. Hence, the input energy to an inkjet heater stack is consumed in several ways. A portion of this energy is transferred to the ink and used beneficially for bubble formation. However, a large percentage of the energy is dissipated in the materials over and under the heater element. Therefore, by minimizing this waste heat into the heater underlayers and/or overcoats, the total required input energy to the heater element can be reduced while still transferring the same amount of energy to the ink.
The realization of ultimate inkjet print quality is influenced by several factors, of which one important driving force is the reduction of droplet size and spacing to the minimum detectable limit of the human eye. However, with current inks, flow features and nozzle materials, ejector and circuit designs, and current thin film materials in heater stacks, printheads are thermally limited due to the extreme heat generated on heater elements. In order to maintain competitive print speeds, the temperature of the heater elements would rapidly rise to >>100° C., eliminating drop-on-demand capability. Conversely, reducing the fire frequency for thermal management purposes would require such a dramatic decrease that the print speed would be extremely slow. Hence, the solution to this dilemma would seem to be to reduce the energy required per heater element fire.
One current approach referred to as a Memjet chip involves a complex process to form a double-sided heater element, i.e. bubbles form on the top and bottom of a heater element on a cantilevered or suspended beam. For instance, see U.S. Pat. No. 7,182,439. While this double-sided heater element does reduce the required energy per fire due to the removal of the thermal mass below the heater element, this approach involves a major departure from the use of conventional inkjet chip fabrication processes and techniques.
Thus, there is a need for an innovation that will assist in achieving an ultra-low energy ejector design while still employing the processes and techniques used in more traditional or conventional inkjet chip fabrication.